/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.socket.sockjs.frame;

import org.springframework.util.Assert;

/**
 * An base class for SockJS message codec that provides an implementation of
 * {@link #encode(String[])}.
 *
 * @author Rossen Stoyanchev
 * @since 4.0
 */
public abstract class AbstractSockJsMessageCodec implements SockJsMessageCodec {

    @Override
    public String encode(String... messages) {
        Assert.notNull(messages, "messages must not be null");
        StringBuilder sb = new StringBuilder();
        sb.append("a[");
        for (int i = 0; i < messages.length; i++) {
            sb.append('"');
            char[] quotedChars = applyJsonQuoting(messages[i]);
            sb.append(escapeSockJsSpecialChars(quotedChars));
            sb.append('"');
            if (i < messages.length - 1) {
                sb.append(',');
            }
        }
        sb.append(']');
        return sb.toString();
    }

    /**
     * Apply standard JSON string quoting (see http://www.json.org/).
     */
    protected abstract char[] applyJsonQuoting(String content);

    /**
     * See "JSON Unicode Encoding" section of SockJS protocol.
     */
    private String escapeSockJsSpecialChars(char[] characters) {
        StringBuilder result = new StringBuilder();
        for (char c : characters) {
            if (isSockJsSpecialChar(c)) {
                result.append('\\').append('u');
                String hex = Integer.toHexString(c).toLowerCase();
                for (int i = 0; i < (4 - hex.length()); i++) {
                    result.append('0');
                }
                result.append(hex);
            } else {
                result.append(c);
            }
        }
        return result.toString();
    }

    /**
     * See `escapable_by_server` variable in the SockJS protocol test suite.
     */
    private boolean isSockJsSpecialChar(char ch) {
        return (ch <= '\u001F') || (ch >= '\u200C' && ch <= '\u200F') ||
                (ch >= '\u2028' && ch <= '\u202F') || (ch >= '\u2060' && ch <= '\u206F') ||
                (ch >= '\uFFF0') || (ch >= '\uD800' && ch <= '\uDFFF');
    }

}
